Information and video displays are used widely in automotive applications, computers, and hand-held devices such as mobile phones and personal digital assistants (PDAs). Light reflecting from instrument panel and center console automobile displays is distracting to drivers and necessitates overhanging dashboard designs that take up valuable interior space. A recent trend toward displaying increasing amounts of information with navigation and communication systems, has compounded the problem of stray light reflection. The glare of unwanted reflected light from mobile phone and automobile instrument panels is an even more severe problem due to the addition of plastic cover windows that are designed to protect the display from damage due to constant handling or cleaning. The reflection of light from plastic display covers is an everyday nuisance to cell phone users who find that the display cannot be read under typical daylight conditions because of strong reflections. Even at night the contrast of displays is reduced by the reflection of light generated by the display itself, or by superimposed reflected images of room lights.
With prescription eyeglasses, reflected light produces overlapping images that hamper the wearer's vision significantly. In particular, reflections from car headlights are a severe problem for eyeglass wearers driving at night. Sunglass wearers have a similar problem during the day where reflections from the lens surface nearest the eye create a distracting image of the wearer's own eye. In addition, reflected light off the front or external lens surface decreases the amount of light transmitted to the eye and thus decreases the optical quality of the lens noticeable to the wearer. This external reflection also produces an annoying shine or glare to an observer of the person wearing the eyeglasses. Glare or glint and decreased light transmission become severe in other optical systems that have multiple reflective surfaces such as the compound lenses found in cameras, microscopes and telescopes.
Converting light from the sun into electricity involves the absorption of the sun's light energy by semiconductor materials such as silicon. The surface of silicon is highly reflective, reflecting a minimum of 30% of the sun's light. AR treatments for silicon solar cells can provide an immediate 30% increase in absorbed solar energy.
Reflected light is a major problem in military, industrial, space and commercial applications that employ infrared (IR) light. Laser communication systems, active and passive imaging sensors, industrial cutting, welding, and marking lasers, and a variety of security devices, typically require durable infrared transmitting windows and optics made of materials such as zinc selenide (ZnSe), zinc sulfide (ZnS or Cleartran®), germanium (Ge), sapphire, silicon, and gallium arsenide (GaAs). Just one surface of a ZnSe window will reflect 17% of the long wave IR (LWIR—7 to 14 micron wavelength) light incident on-axis, a cadmium zinc telluride (CZT) window reflects 21%, and a Ge window or optic will reflect over 36%. The problem gets worse for IR light incident at higher angles off the normal to the window. Such large reflections produce stray light and can lead to superimposed images that can reduce the contrast or even blind security cameras or the night imaging systems found on aircraft, ground equipment, and satellites. Because of the high power levels of many industrial cutting and welding lasers, it is crucial that reflected light from the window and optic materials used is suppressed sufficiently to avoid damage to nearby objects.
The conventional approach to suppressing reflections from optics and windows is to employ multiple thin layers of dielectric materials deposited onto the external surface of the window or optic. This long established method is known in the art as a thin-film anti-reflection (AR) coating. Each deposited layer of material is designed to affect destructive interference for a particular wavelength of light reflecting from the window or optic surface. A great number of thin-film layers are needed to increase the range of wavelengths over which reflections are suppressed. For adequate anti-reflection (AR) over the LWIR range, a typical design would call for as many as 25 layers of material with a total deposited thickness of over 10 microns. In addition, the performance of thin-film AR coatings is limited to applications where light is incident along or near the system axis (normal to the window or optic external surface). For stray light incident off-axis, thin-film coating stacks can produce an increase in reflected light and undesirable polarization effects.
Thin-film AR coatings are typically deposited by high temperature evaporation of the coating materials within a vacuum chamber, a costly process that is problematic for some temperature sensitive materials used in IR cameras or plastics used in visible light applications. Durability and thermal cycling are a concern with thin-film AR coatings where inherent stress and adhesion problems are found due to dissimilar thermal expansion coefficients of the layer materials. Loss of thin-film adhesion from temperature cycling has resulted in catastrophic failure of space-based IR cameras and industrial lasers. Thin-film AR stacks suffer from degradation and short lifetimes in the presence of solar radiation and other harsh environments such as with rain and sand erosion in many military applications. The absorption and dissimilar thermal dispersion found with thin-film AR stacks limits the power attainable with solid state laser designs. Lastly, for display applications, AR coatings are generally not employed due to cost, lifetime, and performance issues such as poor viewing angle, durability, and adhesion loss. Diffuse textured surfaces are sometimes used for display applications such as televisions and computer monitors at the expense of image clarity.
The concept of using surface structures in lieu of thin film coatings to control reflections from optical surfaces has been discussed since the 1970s. The principle is derived from the work of Bernhard et al, who discovered that the eye of the night moth reflects very little light, due to the graded index nature of the moth's cornea. It was hypothesized that the low reflectivity surface of the moth's eye imparted a degree of stealth that protected the moth from its predators, primarily the owl. Wilson and Hutley fabricated the first artificial Motheye surfaces in photoresist using holographic lithography, and demonstrated the concept of Motheye replication using a nickel shim produced by a standard electroforming process. Cowan advanced the fabrication of Motheye textures by closely matching the structure found in nature. In the years since, there has been great interest in the Motheye AR principle, producing several patents and journal publications describing the optical properties and function of these graded index surfaces.
In practice, the surface relief microstructures that make up a Motheye texture, have proven to be an effective alternative to thin-film AR coatings in many IR and visible light applications where durability, radiation resistance, low thermal effects, wide viewing angle, or broad-band performance is critical. These microstructures are built into the surface of the window or optic material, imparting an optical function that minimizes reflections without compromising the inherent properties of the material. Typically, an array of pyramidal surface structures is used that provides a gradual change of the refractive index for light propagating from air into the bulk optic material. Reflection losses are reduced to a minimum for broadband light incident over a wide angular range. In general, these surface relief structures will exhibit similar characteristics as the bulk material with respect to durability, thermal issues (light absorption), and radiation resistance. The problems associated with thin-film coating adhesion, stress, abrasion resistance and lifetime, are eliminated.
To achieve high performance AR with surface relief microstructures, optical phenomena such as diffraction and scattering must be avoided. This requires that the surface structures be fabricated with a periodic spacing smaller than the shortest wavelength employed by the application. In addition, the height and profile of the surface structures should be sufficient to ensure a slowly varying density change.
As general design guidelines, the minimum relief height of the microstructures that make up an AR texture should exceed 40% of the longest operational wavelength, and the distance between structures should be less than 25 to 30% of the shortest operational wavelength to avoid free-space diffraction losses.
For narrow-bandwidth applications such as laser communications, a simpler type of AR surface structure called a sub-wavelength, or “SWS” surface, is used. An SWS is a porous texture that reduces the effective refractive index of the material surface, creating the equivalent of a textbook single-layer, quarter-wave thick, thin-film AR treatment at the design wavelength. Holes in the surface of the material are proportioned to create an effective refractive index equal to the square root of the window material refractive index. For example with an optic made of fused silica glass, the effective index yielded by fabricating a porous SWS texture is 1.21, a value that is not attainable with thin-film coating materials. The depth of the holes in the SWS array is set to one quarter of the design wavelength divided by the effective index. SWS textured windows have been demonstrated that suppress near IR light reflections to levels below the stringent requirements of optical fiber telecommunications (−30 dB, 0.1%).
Both Motheye and SWS AR textures are composed of periodic arrays of surface structures. The periodic nature of these textures produces diffraction effects for light incident at steep angles. For example, a Motheye texture with a 250 nanometer (nm) spacing will diffract visible light in the blue-violet region when the light is incident at an angle greater than 60 degrees off the normal to the window surface. This free-space propagating light is undesirable in certain applications such as the display covers and eyeglasses mentioned above, and for infrared imaging applications that combine visible and infrared scenes. Some mid- or long-wave infrared applications require an AR treatment that also provides a level of camouflage or stealth when observed in visible light or in another region of the infrared spectrum.
There is an immediate need for an optical AR treatment that can provide the performance of Motheye surface textures without producing an observable spread of color at shorter wavelengths due to diffraction. There is also a need for a fabrication process to produce an optical AR treatment without the size limitations and costs associated with optical lithography or thin-film deposition systems.
A process for fabricating non-periodic AR textures that do not produce free-space diffracted light, is disclosed by Schulz et al. in Publication Number US2005/0233083 (Oct. 20, 2005). The texture fabrication process involves the physical ablation of material from the surface of an acrylic plastic using accelerated ions of argon or oxygen generated in a vacuum chamber. The technique disclosed is limited to the fabrication of AR textures in just one type of material—specifically PMMA based plastic—and is not immediately practical for high volume production of eyeglasses or plastic films for displays due to the high costs associated with scaling the vacuum-based process to produce large size parts, sheet film, or higher quantities of parts.